In L1157-B1, the HCN line profiles follow an exponential signature
\(\exp (-V/V_0)\) , with the same \(V_0\)  value for the rotational transitions \(J\) = 1–0 and \(J\) = 3–2, so that the HCN line intensity ratio \(J\) = 3–2/\(J\) =1–0 is almost constant for velocities beyond \(-6\hbox{\hspace{1.99997pt}km\hspace{1.99997pt}s$^{-1}$}\)  (see Fig. REF ). Hence, the HCN excitation conditions are essentially independent of velocity, at least in this velocity range. Similar results were previously reported for CO [1]}.
The similarity of the spectral slopes of HCN and CO between \(-20\)  and \(-5\hbox{\hspace{1.99997pt}km\hspace{1.99997pt}s$^{-1}$}\)  implies that the HCN/CO line flux ratio is constant, and since the excitation conditions of both species are independent of velocity in this interval, it means that the relative abundance ratio is also independent of velocity in this velocity range.
